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Home , Astronomical object, Crab Pulsar, Guest star (astronomy)

Introduction to Supernovae

According to the In any case, no European record of the event has Annals of the Sung Dynasty (the ever been found. Sung-shih), on the first day of the chi-ho Since the appearance nearly a millennium ago of reign period, during the 5th month, on the the Sung “guest ” there have been only two chi-chou, a “guest star” appeared to the south other similar objects seen in our . One oc- east of Tian-kuan. curred in 1572 in the Cassiopeia. The guest star was so This was observed by the Danish Ty- bright that it could be cho Brahe and bears his name. It became bright seen during the day- enough to be visible in full daylight. The other , and it remained star appeared in the constellation Ophiuchus in so for 23 days. Af- 1604 and was studied by Tycho’s student and col- An ter that, it gradually laborator, Johannes Kepler, though it was seen un- dimmed, finally fading known artist earlier by several other people. Kepler’s star, while may have recreated the from visibility after two years. not as bright as Tycho’s, was still as bright as Jupi- appearance of the Crab Japanese records also mention the ter. Since the appearance of Kepler’s star, no oth- in star. 1054 on the side of ers have been seen in the Galaxy. a cave wall in Chaco At around the same time, half a This does not mean, however, that no additional Canyon, New Mexico. world away from China in what similar objects have been observed. In 1885 a new we now call Chaco Canyon, in star appeared in the center of the Milky Way’s Northern New Mexico, an ancestor of today’s companion galaxy M31, in the constellation of Hopi and Navajo painted what appears to be a Andromeda. It reached a peak brightness of 6-7th “guest star” into a protected rock overhang. The , making it easily visible in small tele- star is shown next to the crescent , and scopes against the background glow of the galaxy a hand print, perhaps that of the artist, is also itself. The object is important for historical rea- painted on the rock. Though this record is not sons because it was used to argue, incorrectly, that as detailed as that of the Sung , the the great spiral nebula of Andromeda was a new orientation of the moon and the guest star is , like our , in the process of what it would have been on that day when the forming within our own galaxy. The astronomer Sung reported their guest star, strongly suggest- Harlow Shapley, in a famous 1922 debate with ing that the two are the same. Heber Curtis on the nature of the spiral nebulae, Such an impressive object, recorded in dispa- claimed that the appearance of the guest star in rate cultures around the globe, must have been Andromeda was due to a “new ” just begin- visible in Europe as well as Asia and North ning to turn on. His argument was later shown to America. However, the date given in the Chi- be wrong. Edwin Hubble measured the distance nese annuls, by our modern reckoning, would to the Andromeda nebula and proved it was ex- have been July 4, 1054. At that time Europe- tremely far away and, in fact, an independent sys- ans were in the throes of the Dark Ages, and tem of – a galaxy on a par with our own the Norman Invasion was just a few years Milky Way. away. Perhaps they were too occupied Though these guest stars are rare with worldly concerns to mark down events in any given galaxy, the the appearance of a celestial visitor, or perhaps whatever record existed has been lost. The is located just above the star mark- ing the tip of the lower horn of , the Bull.

If you point a telescope toward the patch of described in the Chinese records from 1054, just a few degrees north and east of , the “eye” of Taurus, the bull, you will find a faintly glowing cloud. This is the Crab Nebula. It is the remains of a star that exploded some 7000 years ago. The explosion was seen on contains many, many . With the ad- only 1000 years ago because it was vent of large telescopes in the 1920s and 30s it so distant that its light required 6000 years was soon noticed that guest stars could be seen to reach us; the Sung and Chaco Canyon quite often if one looked at many galaxies. The inhabitants were seeing the explosion 6000 fact that the guest stars were nearly as bright as years after it happened. The Crab Nebula the galaxies in which they occurred meant that is a , the debris from they were enormously energetic. Their great an exploded star. It is still expanding brightness and release of energy prompted the today at more than 1000 km/s, (for astronomer Fritz Zwicky to dub them superno- more about supernova expansion vae, because they appeared similar to, but far see p.11) having slowed from an brighter than, the “novae” seen in our galaxy. initial expansion speed of more than Supernova is the name by which we still call 10,000 km/s. Inside the nebula is them today, though we now know they have the Crab , the compact nothing in common with novae except a name: remnant of the core of the supernovae are exploding stars, whereas novae exploded star. The pulsar is a are the much smaller explosion of the atmo- highly magnetized, rapidly sphere of a white that is acquiring spinning , a class matter from a nearby binary companion star. of object that is among the (For more about supernova life cycle, see p.8.) most bizarre found in nature. In an ironic twist, recent observations of super- A mere teaspoon of the crab novae similar to the one seen in Andromeda pulsar would weigh in 1885 have allowed us to measure the vast more than a billion size and expansion rate of the universe. To our tons (for more about great surprise, these extremely distant superno- neutron stars see vae indicate that the expansion is accelerating, p.22.) rather than slowing down. These observations In the remainder indicate that approximately 70% of the energy of this education in the universe is something never before unit, you will ex- observed, with properties heretofore only plore the amazing properties imagined in the most speculative of our of supernovae and neutron stars. theories of nature. Far from showing You will also begin to learn that the universe is small, as Shapley ar- about some of the gued, supernovae have shown us that tools scientists use the universe is not only vast, but to understand much stranger than we had them. imagined. Why stars explode

The stars in the sky seem eternal and unchanging.

But that’s an illusion. Like Initially, the star fuses hydrogen into helium. Like all things, stars are born, live out ash in a fire, the helium builds up in the core, but it their lives, and eventually die, doomed does not fuse because helium takes a lot more pres- to fade away. Stars like the Sun, which have sure and heat than hydrogen does to fuse. If the star a relatively low , age gracefully and die is massive enough, though, it can ignite helium fu- quietly after billions of years. But massive stars, sion in its core. The helium fuses into carbon, which with more than ten or so the mass then starts to pile up in the core. In very mas- of the Sun, “do not go gently into sive stars this process repeats again and H that good night, but instead again, fusing lighter elements into rage, rage against the dying of He heavier ones: hydrogen to he- the light”. They explode in lium, helium to carbon, carbon a catastrophic detonation, C to neon, neon to , oxy- sending their outer layers O gen to silicon, silicon to iron. screaming outwards at a Si The star’s core starts to look few percent of the speed Fe like an onion, with layers of light: what astronomers nested inside one another. call a supernova. At every step, the process The seeds of a star’s - ulti generates more heat, and the mate destruction are plant- fusion goes ever faster. A star ed deep in its core, where its may fuse hydrogen into helium energy is generated. Stars are for millions or billions of years, giant balls of gas, and when a gas but by the time it starts to fuse sili- is compressed it heats up. Be- con into iron, it may take mere cause stars are so big they have Near the end of a massive star’s life, the fusion occurs in shells days. As iron piles up in the a lot of , so at the core around the core, like the layers of an onion. core, the star is headed for di- of a star the pressure is intense. saster. This means they get very hot, hot enough to smash Why? Because up until iron, all the fusion reactions together atomic nuclei. And when nuclei collide, have produced energy in the form of heat. That heat they can stick together in a process called fusion. holds the star up. However, iron is different. It takes This process releases a lot of energy (in fact, it’s energy to fuse iron into heavier elements, and this what makes hydrogen bombs explode), which energy must come from the star itself. When enough heats up the core. In a stable star like the Sun, iron builds up in the core, the pressure becomes great the inward crush of gravity is balanced by outward enough that it starts to fuse. This robs energy from pressure caused by the heat. the star, cooling it. Worse, the fusion of iron eats Already we see that the mass of the star is impor- up copious amounts of electrons, and the of tant: it provides the gravity needed to compress these electrons was helping to hold up the star too. the core. The higher the mass of the star, the more When iron starts to fuse, things go bad fast. The iron the core is compressed, and the hotter it can get. core collapses, since the heat and electrons holding Fusion reactions depend strongly on temperature; it up get used to fuse the iron. In a thousandth of a the higher the temperature, the faster the reaction second the tremendous gravity of the core collapses proceeds. As we’ll see, this is critical later in the it down from thousands of kilometers across to a star’s life. ball of compressed matter just a few kilometers in diameter. This is a bit like kicking the legs out And what of the core? Like the life of the star from under a table. Just like when Wile E. Coy- itself, the fate of the core depends on its mass. ote suddenly realizes he is no longer over solid In relatively low-mass stars like the Sun, the star ground and starts to fall, the outer layers of the never explodes at all. The core is not massive star come rushing down. They slam into the enough to fuse helium, so helium simply builds compressed core at a up. Or perhaps helium significant fraction of does fuse, but then the speed of light. the star is not massive enough to fuse the re- This does two things: sulting carbon. In any it sets up a huge re- event, the outer layers bound, sending the of the star are blown outer layers of the star off by a solar wind over back out, and also re- millions of years, and leases a vast number the naked core, un- of neutrinos, sub- able to generate its own atomic that heat, simply cools and carry away most of the fades away. A star that con- energy of the collapse. The These pictures shows the location of Supernova 1987a before it exploded (left), and during the explosion (right) sists of this revealed core is gas from the outer layers called a . absorbs only a small frac- tion of these neutrinos, but that’s still a lot of If the core is more massive, between 1 and 3 energy: it’s like lighting a match in a fireworks times the Sun’s mass then things are different. factory. The outer layers of the star explode up- The pressure from the collapse slams electrons wards, and several solar of doomed star into protons, creating neutrons. The core shrinks (containing the elements that were produced to a size of a few kilometers across, and is com- before the explosion) tear outwards at speeds of prised almost totally of these neutrons. The col- many thousands of kilometers per second. lapse is halted by the neutrons themselves, which resist the pressure. Not surprisingly, this object is As the star explodes, the expanding gas is so hot called a neutron star. that it can undergo temporary fusion, creating elements as heavy as uranium. This, plus other And for more massive cores? Even the neutrons radioactive elements created in the explosion, cannot resist the pressure created by more than dumps even more energy into the gas, causing it about 3 times the Sun’s mass when it collapses. to glow. The expanding gas is called a supernova The core implodes, and nothing can stop it. Its remnant; it will expand for hundreds of thou- gravity increases hugely, and anything that gets sands of years, eventually cooling and becoming too close will be drawn in, even light. It has be- so thin it merges with the tenuous gas between come a . the stars. Sometimes the gas This is more than just the- from the remnant will hit and ory. By studying superno- mix with gas that is forming vae, supernova remnants, new stars, seeding it with the and other exotic objects, heavy elements formed in astronomers have discov- the explosion. The iron in ered all this and much your blood and the calcium more. If you want to con- in your bones were formed tinue reading about this in the supernova explosion and get more information, of a massive star millions of check out the Resources years before the formation of list in the last section of the Earth itself. David De Martin (http://www.skyfactory.org), Digitized Sky Survey. this poster. Description of the Illustrations

The following images provide an artistic rendering of emission from a superno- va remnant in three different energy regimes: optical, x-ray and gamma radiation. While the overall shape of the remnant is the same in each energy band, the processes underlying the emission can be quite different.

The first and second images show emission mostly from gas at the edge of the remnant. As the ejected material from the supernova encounters gas that already existed around the star, it creates shock waves, similar to the way a supersonic jet makes shock waves in the air. Inside these shocks the gas ejected from the exploded star is slowed, compressed, and heated to millions of degrees. This happens because, as the expanding supernova material col- lides with the existing gas surrounding the star, its kinetic energy – the energy of its motion outward from the supernova – is converted to random mo- tions, called turbulence. The high temperature of the gas means the atoms are moving very rapidly, which leads to very energetic collisions between them. The collisions are so energetic that they kick electrons completely off of the atoms, that is, the atoms become ionized. When gas is this hot, the atoms bounce off each other at high speeds, generating X-rays. Image 2 in the large panels on the front of the poster depicts this sort of emission. Optical view

As the remnant emits X-rays it loses energy. After all, X-rays are a form of radiation, like optical light, so they carry away a lot of the energy of the supernova into space. That loss of energy causes the remnant to cool. After several tens of thousands of years the outer shocked part of the remnant has cooled to only a few thousand degrees. Gas at this temperature is not hot enough to produce strong X-ray emission, but it is hot enough to excite (give energy to) the atoms within it. When an atom absorbs energy, its electrons jump from one energy level to another, like someone going up a staircase. After some time, the electron then falls back down to a lower level, and emits energy at a very specific wavelength. This type of emission is called line emission. In a low density but hot gas like that of an old supernova remnant, the emission lines typically seen are from excited and ionized forms of hydrogen, oxygen, nitrogen, sulfur, and other kinds of atoms. This can make supernova remnants appear to our eyes to glow with characteristic colors such as red or green. This lower-temperature gas tends to be from the outer parts of the supernova remnant. The inner part of the remnant is still filled with million degree x-ray emitting gas. This is because the density inside the remnant is much lower than in the outer shocked parts, and the lower density causes the inner part of the remnant to cool much more slowly. In fact, it takes more than a mil- lion years for the hot bubble inside a supernova remnant to cool completely. Because these two types of emission depend on the temperature of the gas X-ray view emitting them, they are sometimes collectively called thermal emission. The last frame in the poster depicts a different type of radiation process entirely. This image shows the emission of gamma rays, the most ener- getic type of electromagnetic radiation. The gamma rays are not emitted by the gas itself because even a million degrees is not hot enough to produce gamma rays. Instead, the emission is produced by the interaction of the electrons in the remnant with the magnetic field of the compact neutron star in its center. This neutron star is the remains of the core of the star that collapsed. The neutron star is fantastically small and dense: while it’s only a few kilometers across, a single teaspoon of neutron star material weighs as much as 100 million adult elephants, or a mountain a kilometer high! It spins rapidly, up to thousands of times per second, and can possess extremely strong Gamma-ray view magnetic fields. As the star’s spin sweeps its magnetic field through the remnant, it picks up charged particles such as electrons like a fisherman’s net sweeps up fish. These particles are accelerated to speeds close to the speed of light as they ride along with the magnetic field.

When charged particles are accelerated like this, the neutron star itself. they emit radiation. However, the radiation is very Examples of this kind different from the emission described for the first of remnant are the Crab two panels. This type of emission, called synchrotron Nebula and the Vela su- emission, does not come out at one specific energy. pernova remnant. Rather it is seen in a very broad, nearly flat continu- However, that’s not al- ous spectrum, spanning all the way from radio waves ways the case. If the to gamma rays. Furthermore, in contrast to emis- original supernova ex- sion lines and thermal X-rays, synchrotron emission plosion is off-center Crab nebula will not diminish as the remnant cools. It depends (not a perfectly expand- only on the spinning, magnetized neutron star, not ing sphere), it can give a “kick” to the neutron star, on the temperature of the gas. Because of this pe- basically acting like a rocket. The neutron star can culiar property, is referred to be ejected from the explosion at very high speeds, as non-thermal emission. Spinning magnetized neu- hundreds of kilometers per second. After hundreds tron stars, also called because they emit pulses of years, the neutron star can actually leave the su- of electromagnetic radiation, are seen to slowly lose pernova remnant. In those cases, the main source of their spin over long periods of time. There is a lot of energy for the gas is gone. These supernova remnants energy in the spin of such a massive object, and if it’s still glow from their own heat, but they tend to be slowing, that energy has to go somewhere. Careful (but are not always) much fainter than pulsar-ener- study of the energy emitted by supernova remnants gized remnants. shows that the energy emitted by them matches the energy lost by the slow- By examining supernova remnants at these different ing spin of the neu- energies, astronomers learn different things about tron star. Therefore, them. Their ages, energy, chemical content, and astronomers conclude much more can be determined by multi-wavelength that late in the life of observations. That’s why NASA, ESA, and other a supernova remnant, space agencies continue to launch high-energy ob- its energy comes from servatories; it’s only when seen at these different en- Vela Nebula ergies that the Universe reveals its secrets. Timeline Description

When the supernova blast wave breaks through the surface of the doomed star, it sets into motion a series of events and processes which will literally shape the fate of the expanding gas over the ensuing millennia.

The outer layers of the star explode outward due to the blast wave created when the core of the star collapses. The heat and pressure from this blast are so high that nuclear fusion can actually take place in the material. Elements up to uranium on the periodic table are created, many of which are radioactive. Isotopes of cobalt, aluminum, tita- nium, and nickel are all produced in the fireball. Expanding gas normally cools, but as these radioactive elements decay they produce gamma rays. The gas absorbs this radiation, heating it. After a week or two, much of the light emitted by the supernova remnant is due to this heating from radioactive decay.

As time goes on, the remnant continues to expand. During the first stage of its life, the gas in the remnant is dense enough that it is opaque; we only see the outer part of the expanding cloud. But as it expands its volume increases, so its average density drops. Eventually, like a fog clearing, the gas becomes transparent to visible light. We see deeper into the remnant, and after a few years the spinning pulsar—the collapsed core of the star— becomes visible. The gas inside the remnant is energized by the magnetic fields of the pulsar (see above). That gas is diffuse and to our eyes glows with a bluish hue. The gas in the outer parts of the remnant, however, has been compressed into thin filaments and ribbons by the shock wave, and is dominated by line emission. This gas glows mostly red and green.

Over many thousands of years, the pulsar-driven gas from the inner part of the nebula has caught up with and merged with the outer gas. The pulsar spin has slowed, its energy given to the expanding remnant. All that’s visible now are the shocked filaments, thin wisps of gas. The remnant resembles a giant spider web.

Eventually, over tens or even hundreds of thousands of years, the gas in the remnant mingles with the gas between the stars in the Galaxy. The remnant is no longer a discrete entity, but instead has merged with the . Stars form from such re- gions of gas and dust, and are thus enriched by the supernova explosion: heavy elements such as calcium, silicon, uranium, iron, and many others seed the nascent stars. These heavy elements may be necessary to form , and in fact the iron in our blood and the calcium in our bones were formed in the heat and fury of some ancient supernova. We owe our very existence to some long-dead star, whose remains deposited its contents into the cloud of gas and dust that eventually became the Sun, the Earth… and us. What is the Gamma-ray Large Area Space Tele- What is XMM-Newton? scope (GLAST)? XMM-Newton is an X-ray The Gamma-ray Large Area launched into Earth (GLAST) is a NASA satellite planned for launch in orbit on December 10, 1999 2007. GLAST is part of NASA’s Science Mission by the European Space Agen- Directorate. Astronomical like GLAST are cy (ESA). XMM-Newton is a designed to explore the structure of the Universe, ex- fully-functioning observatory, amine its cycles of matter and energy, and peer into carrying three very advanced the ultimate limits of gravity: black holes. GLAST X-ray telescopes. They each is being built in collaboration between NASA, the contain 58 high-preci- U.S. Department of Energy, France, Germany, Italy, sion concentric mir- Japan, and Sweden. The project is managed from rors, nested to offer NASA’s Goddard Space Flight Center in Greenbelt, the largest collecting Maryland. GLAST detects gamma area possible to catch rays, the highest energy X-rays. Unlike many other light in the electromag- telescopes, which only make images of the netic spectrum. objects they observe, XMM-Newton takes GLAST both images and spectra. This means it can measure the energy of the X-rays emitted by an astronomical object, which allows scientists to determine many of its physical characteristics in- cluding temperature, composition, and density.

Activity 1 - Biography of a Supernova This poster acompanies an Brief overview: educator guide which contains Students will write an essay describing the the following activities: evolution of a supernova remnant based on pictures from the Supernova Poster. Grades: 7 – 12 Activity 2 – Crawl of the Crab

Brief overview: Students use two images of a supernova remnant Activity 3 – At the Heart of a Supernova Explosion separated in time by several decades to determine the expansion rate of the glowing gas. Brief overview: Grades: 8 – 12 Students investigate the magnetic fields and other properties of pulsars and compare them to the Earth’s magnetic field. Get the guide at Grades: 8 – 12 http://xmm.sonoma.edu/edu/supernova

Credit This poster has been developed as part of the NASA Education and Public Outreach (E/PO) Program at Sonoma State University, under the direction of Professor Lynn Cominsky. Contributors to this education unit include Professor Lynn Cominsky, Dr. Kevin McLin, Dr. To learn more about GLAST, Philip Plait, Sarah Silva, and Aurore Simonnet. We would also like to acknowledge input from and XMM-Newton education lots of other nice people too. and public outreach, visit:

http://glast.sonoma.edu or http://xmm.sonoma.edu